MINOR: checks: allow external checks in backend sections

Previously, external checks required to find at least one listener in order to
pass the <proxy_address> and <proxy_port> arguments to the external script.
It prevented from declaring external checks in backend sections and haproxy
rejected the configuration.

The listener is now optional and values "NOT_USED" are passed if no listener is
found. For instance, this is the case with a backend section.

This is specific to the 1.6 branch.
MEDIUM: checks: provide environment variables to the external checks

The external command accepted 4 arguments, some with the value "NOT_USED" when
not applicable. In order to make the exernal command more generic, this patch
also provides the values in environment variables. This allows to provide more
information.

Currently, the supported environment variables are :

PATH, as previously provided.

HAPROXY_PROXY_NAME, the backend name
HAPROXY_PROXY_ID, the backend id
HAPROXY_PROXY_ADDR, the first bind address if available (or empty)
HAPROXY_PROXY_PORT, the first bind port if available (or empty)

HAPROXY_SERVER_NAME, the server name
HAPROXY_SERVER_ID, the server id
HAPROXY_SERVER_ADDR, the server address
HAPROXY_SERVER_PORT, the server port if available (or empty)
HAPROXY_SERVER_MAXCONN, the server max connections
HAPROXY_SERVER_CURCONN, the current number of connections on the server